Create an event on epic actions
Currently when an epic os open/closed/reopened/commented, no event is created for this action (except when commenting an epic, but in this case the event is created without group reference, so it's not very usable). We should start creating these events if we want to show them on group/dashboard activity lists (related to comment https://gitlab.com/gitlab-org/gitlab-ee/issues/7385#note_207997970)
This is needed for issue https://gitlab.com/gitlab-org/gitlab-ee/issues/7385.